A. Field of the Invention
The present invention relates to magnification viewers worn by surgeons and dentists. In particular, it relates to a compact, light-weight, comfortable-to-wear, high magnification viewer having an extremely wide field of view and good image quality.
B. Description of the Prior Art
Magnification viewers are worn by dentists and surgeons for extended periods of time during clinical procedures so as to provide clarity of view while avoiding a "hunched-over" position that can result in debilitating neck and back strain, which can have an adverse effect on the success of the operation. By permitting the clinician to operate at a greater working distance from the patient, higher magnification viewers also reduce the clinician's exposure to aerosols.
Because clinicians use magnification viewers during surgery and other procedures requiring manual precision, it is important that they be light-weight, comfortable, and have good clarity and wide field of vision while providing high magnification.
Clinical magnification viewers are generally made according to the "Galilean telescope" design, having a single objective lens and a single eyepiece lens. Galilean telescopes are characterized by relatively narrow fields of view which are mainly limited by the diameter of the objective lens. The basic Galilean design, however, produces substantial chromatic aberration ("coloring") and, hence, poor image quality.
The magnification, or power, of a Galilean telescope is proportional to the focal length of the objective and inversely proportional to the focal length of the eyepiece. Overall viewer length is proportional to the sum of the focal lengths of the objective and eyepiece.
Since the viewer should be kept as short as possible to reduce torque on the nose and wearer discomfort, an eyepiece with a shorter focal length is usually employed when an increase in magnification is desired. However, to retain a good field of view without vignetting, the diameter of the objective must be increased. If this is done while keeping the focal length of the objective the same, the "speed" of the lens increases, which results in a lower resolution quality. It also mandates an excessively large package. One method of overcoming the "speed" problem is to use a more complicated objective lens, though at the cost of greatly increased weight and strain and discomfort to the wearer.
The so-called Kellner design (from Kellner, U.S. Pat. No. 1,197,742 "Lens System") in general use today contains a heavy doublet objective and a singlet eyepiece lens. While image quality is adequate at lower magnifications, at higher magnifications, excessive coloring results in poor image quality. Moreover, the field of view is relatively limited.
It is known that image quality in prior art magnification viewers can be enhanced by the use of "very high index flint glass". However, this method has not been in general use, since "very high index flint glass" makes the viewer too heavy for practical use.
Finally, prior art magnification viewers require lens mounting barrels of differing sizes in order to provide a wide range of focusing or working distances. As such, the manufacturing costs for prior art viewers are relatively high.